Low-temperature fuel cells based on polymer electrolyte membrane (PEM) technology are being discussed as environmentally friendly and efficient energy converters for portable, mobile and stationary applications and are already being used commercially for the first time. They convert hydrogen and oxygen into electrical direct current at temperatures ranging from just above freezing point to approximately 90° C., yielding water as the only by-product.
At the heart of a PEM fuel cell is a membrane electrode assembly (MEA) comprising a polymer electrolyte membrane with a gas diffusion electrode on either side and an electrocatalyst layer (e.g. of platinum) disposed therebetween. The gas diffusion electrodes typically consist of a solid, gas-permeable and electrically conductive substrate material (e.g. carbon fabric or paper).
A good permanent bond must exist between the membrane and the gas diffusion electrodes in order to achieve good proton conductivity. This bond has hitherto been established mainly by compressing the membrane and the electrodes at temperatures in excess of 100° C. in a hot press. Such a method is known e.g. from the above mentioned article.
The two electrodes can be simultaneously bonded to the membrane in a single pressing operation. Alternatively a first electrode can also be bonded to a first side of the membrane in a first pressing operation and then a second electrode can be bonded to another, opposite side of the membrane in a second pressing operation. This produces a permanent bond, i.e. even after removal of the pressing pressure and the pressing temperature which may be present, the bond remains intact throughout the lifetime of the membrane electrode assembly.
The common feature of these methods is that the membrane and the at least one electrode are pressed together by the pressing together of two plates of a press which are in direct contact with the electrode(s) and the membrane.
The attendant problem is that these plates often cannot be aligned exactly parallel, resulting in an inhomogeneous application of pressure to the membrane and the electrode(s), i.e. different pressing pressures along the surface of the membrane and electrode(s), and therefore inhomogeneous compression of these components. The same effect occurs in the event of unevennesses in the plate material, membrane or gas diffusion electrodes. This effect results in displacements of the materials in the press, uneven contact pressure and therefore uneven bonding between the electrodes and the membrane or even warping or destruction of the membrane and electrode material when the pressing pressure is removed. This significantly reduces the proton conductivity between the membrane and the electrodes.